ExbB

Structure
ExbB consists of three transmembrane domains (spanning from residues 16-39, 128-155 and 162-194), with two large portions of the protein representing the majority of the protein in the cytoplasm and two smaller portions in the periplasm.

ExbB can exist in the TonB system either in a complex with ExbD (to a ratio of 3.5:1) or on its own (where no ExbD is present), which has been suggested to play a part in the diverse roles of TonB.

ExbB has a similar sequence and physiological structure to TolQ and is therefore thought to be evolutionarily linked.

Function
ExbB is essential for TonB-dependent energy transduction as the absence of ExbB prevents TonB responding to the proton motive force, as well as the change of the high-affinity association of TonB for the outer membrane to the cytoplasmic membrane.

Loss of ExbB function can be partially replaced by TolQ and vice versa - reduced activities of either of these proteins via a mutant form can be reversed by introducing double mutants.